This invention is related to highly localized Infrared (IR) spectra on a sample surface utilizing an Atomic Force Microscope (AFM) and a variable wavelength pulsed IR source.
IR spectroscopy is a useful tool in many analytical fields such as polymer science and biology. Conventional IR spectroscopy and microscopy, however, have resolution on the scale of many microns, limited by optical diffraction. It would be particularly useful to perform IR spectroscopy on a highly localized scale, on the order of biological organelles or smaller, at various points on a sample surface. Such a capability would provide information about the composition of the sample, such as location of different materials or molecular structures. Conventional infrared spectroscopy is a widely used technique to measure the characteristics of material. In many cases the unique signatures of IR spectra can be used to identify unknown material. Conventional IR spectroscopy is performed on bulk samples which gives compositional information but not structural information. Infrared Microscopy allows collection of IR spectra with resolution on the scale of many microns resolution. Near-field scanning optical microscopy (NSOM) has been applied to some degree in infrared spectroscopy and imaging. While there have been some promising laboratory results, there is still investigation and discovery required to enable a sensitive, and reliable commercial instrument. To our knowledge no widely available instrument provides routine IR spectra with resolution below the diffraction limit. Recently, one of the inventors has developed a technique based on use of an AFM in a unique fashion to produce such localized spectra. This work was described in a publication entitled “Local Infrared Microspectroscopy with Sub-wavelength Spatial Resolution with an Atomic Force Microscope Tip Used as a Photo-thermal Sensor” Optics Letters, Vo. 30, No. 18, Sep. 5, 2005. Those skilled in the art will comprehend the details of the technique in the publication but the technique will be described briefly herein for clarity. The general technique is also referred to as Photo-Thermal Induced Resonance, or PTIR.
Many AFM designs are known in the art, one common design is illustrated in FIG. 1. A cantilever with probe tip 2 is brought into proximity with a sample 3. As the probe tip interacts with the sample it influences the motion of the cantilever. This motion is often detected by a light beam 1 that is directed to the cantilever 2 which reflects the beam to a photo-detector 4. Deflection of the cantilever vertically due to interaction with the sample causes the beam spot to move on the detector, generating a difference signal from the detector quadrants. This type of AFM setup is called an optical lever arm, and commercially available AFMs using this technique can measure deflections of the lever on a sub-angstrom scale. Typically the photodetector is a 4-quadrant type, allowing motion of the vertical and lateral motions of the cantilever. Many other optical detection schemes have been employed including optical interferometry and diffraction. There are other options to detect the deflection of the cantilever that can alternately be used, such self sensing cantilevers that employ piezoresistive, piezoelectric, capacitive, and inductive readouts, for example. Any detection technique that is sensitive to the motion of the cantilever may be suitable. Preamplifiers and/or other signal conditioning electronics are often used to amplify the detector signal before data acquisition and processing. A scanner (not shown) is typically used to generate relative motion between the probe tip and sample. The scanner can create this motion by moving the sample, the probe tip or a combination of both. Scanners are typically made from one or more piezoelectric elements, but suitable scanners can also be made from actuators employing electrostatic, electrostrictive, magnetostrictive, inductive, voice coil and other motion mechanisms. Other actuators using other scanning mechanisms will also work as long as they can generate probe and/or sample motion over the scan ranges desired in response to an input signal. Often the scanners contain mechanical flexures to guide and or amplify the motion of the actuator. Feedback systems are typically employed to servo the sample and/or or tip up and down in response to height variations of the sample to maintain a desired interaction between the probe and the sample. This vertical servo signal vs. lateral position creates a topographical map of the surface which can achieve atomic resolution. A wide variety of variations of the AFM exist with different types of probes and so on for measurements other than topography. For instance, in a co-pending application by some of the inventors of this application, a version of an AFM configured to measure thermal properties of a surface is described.
The AFM set-up used for the published work on IR spectroscopy is shown schematically in FIG. 2. In this set-up, the sample 3 is mounted on a ZnSe prism, or prism made from other suitable materials, which does not absorb the radiation of interest. A pulsed IR source 9, in this case a Free Electron Laser beam, is directed into the prism. The prism is made at an angle such that the beam is in Total Internal Reflection in order for the beam to be propagative in the sample and evanescent in the air. Thus only the sample is significantly exposed to the laser radiation, and the AFM probe 2 is minimally exposed to beam 9. The Free Electron Laser (FEL) is an IR source that is both variable in wavelength and has a pulsed output. Free Electron Lasers are large expensive facilities, available at only a few institutions in the world. The FELs are also shared facilities such that each user only may access a limited amount of beam time per year. The probe 2 is placed at a point on the sample by the scanner 5 and is held at an average height by feedback electronics 6. Both the vertical and lateral deflection signal as well as the feedback signal, are may be monitored at 7.
Referring to FIG. 3, when the FEL is pulsed, the sample 3 may absorb some of the energy, resulting in a fast thermal expansion of the sample as shown in the Figure. This has the effect of a quick shock to the cantilever arm 2, which if the ability of the cantilever to respond to this shock is slower than the shock will result in exciting a resonant oscillation in the cantilever arm. The resonant oscillation decay or “cantilever ringdown” 12 is shown in FIG. 3. Because the absorbed energy is ideally contained within the sample, this shock is due primarily to rapid sample expansion as minimal IR energy is absorbed by the cantilever itself. Although the probe is kept in contact with the surface by the feedback electronics, the resonant signal is too fast for the feedback electronics, but can be observed directly from the photodetector 4. Thus the cantilever rings down in the manner shown in FIG. 3 while still in contact with the surface, an effect called “contact resonance”. The absolute deflection, amplitude and frequency characteristics of the contact resonance vary with the amount of absorption as well as other properties, such as the local hardness, of the localized area around the probe tip, for example by analyzing the ringdown and/or the Fourier transform (FFT) 13 of the ringdown events. Also, depending on the direction of the expansion, vertical resonances, lateral resonances or both can be excited. If the tip is to the side of the absorbent material, this will typically cause a stronger lateral response in the cantilever. By repeating the above process at varying wavelengths of the FEL, an absorption spectra on a very localized scale is achieved. By scanning the probe to various points on the sample surface and repeating the spectra measurement, a map of IR spectral surface characteristics can be made. Alternatively, the wavelength of the FEL can be fixed at a wavelength that is characteristic of absorption of one of the components of the sample. The probe can then be scanned across the sample surface and a map of the location of that component can be generated.
Although the set-up as described produced positive results, there is no real possibility of commercializing the set-up as published. First, the IR light source used, the Free Electron Laser is a very large and expensive facility and only a few exist in the world. Alternative benchtop sources of IR radiation have been limited by one or more characteristics that have made them unsuitable for a widely available instrument. Picosecond OPO pulsed lasers have been used, but suffer from very high costs and low pulse repetition rates, often a few tens of Hz. Broadband IR sources like glowbars are sufficient for bulk IR spectroscopy, but have insufficient optical power density for micro and nanoscale applications unless used with unacceptably long acquisition and averaging times. CO2 lasers have limited wavelength range and do not address a wide enough bandwidth to cover the “fingerprint region.”
The apparatus described in the publications suffers from other limitations beyond the expensive and stationary IR source. The apparatus employs a bottoms-up illumination scheme that requires a sample to be placed on a specially fabricated IR transmitting prism. In addition to being costly and easy to damage, this arrangement requires special sample preparation techniques to prepare a sample thin enough such that the IR light can penetrate the sample to reach the probe. Further, the actual signals generated can be small thus requiring averaging of the signal and limiting the bandwidth of the technique. More sensitivity would be required to address a wider range of potential samples. Also, up to this point, the technique is not quantitative in terms of the amount of absorption that occurs in the local area. Determining the absorption from the contact resonance amplitude as well as the measurement of some thermal properties of the sample would improve the ability of the technique to identify materials. Thus a variety of issues must be addressed in order to take the published technique from a laboratory set-up to a commercial analytical instrument. The present invention addresses the commercialization issues.